Recently, FRP bonnets for automobiles have been developed for a first purpose of weight lightening, and various structures are proposed aiming mainly to increase the strength and the rigidity of a required portion (for example, JP-A-2003-146252). An FRP bonnet is usually formed from an outer member with a high rigidity provided at a surface side and an inner member joined to the back-surface side of the outer member which has a stiffener structure with, for example, a hat shape in cross section. The high rigidity of the outer member is obtained, for example, by forming it thick or forming it as a sandwich structure in which a core material is interposed between FRP skin plates.
However, in a case where an outer member is formed thick, the weight thereof greatly increases, an advantage achieved by a bonnet with FRP is lost. On the other hand, if a sandwich structure is employed, although a high advantage of weight lightening can be obtained, when a large-area member such as a bonnet is molded as a sandwich structure over the entire area, it can be molded relatively easily in a case where the core material is thin, but the core material has to be formed thick in order to obtain a high rigidity, and therefore, the molding is associated with a fairly great difficulty from the viewpoint of giving a shape to the core material and processing the core material.
Further, although it is considered that the outer member is formed thin by designing the rigidity of the FRP inner member high and making the inner member exhibit a main rigidity of the bonnet, in this case, a line-like deformation due to a difference in rigidity, a so-called bonding line, frequently appears at a portion on a surface-side design surface of the outer member which corresponds to a joining portion between the thin outer member and the high-rigidity inner member, and the design feature of the surface may be damaged.
By the way, separately from the above-described problems in an FRP bonnet concerning weight lightening, providing of a required rigidity, easiness of molding, bonding line, etc., recently, enhancement of safety at a collision accident etc. has been required for an automobile, in particular, enhancement of performance for protecting a pedestrian at a collision accident etc. has been required, and in order to enhance this function as highly as possible, an excellent impact absorbing function has been required. When an automobile collides with a pedestrian, the pedestrian receives an impact load on the leg or the head against a front portion, a bonnet, etc. of the automobile, and particularly, in order to reduce deadly accidents, it is said that it is inevitable to reduce a damage to the head. With respect to this reduction of the damage to the head, a regulation value as an impact relaxation performance of a bonnet is being standardized, and in particular, a head injury criterion (HIC), which is calculated by an acceleration received by the head and the duration thereof, is required to be suppressed at a predetermined value or less.
Accordingly, paying attention to the above-described circumstances, it could be advantageous to provide a bonnet made from FRP for an automobile which can increase a rigidity of a required portion without damaging moldability while maintaining an advantage of weight lightening of the bonnet by making it from FRP, can maintain a design feature of the surface at a good level, and in addition, can sufficiently enhance the performance relating to protection of a pedestrian at a collision accident etc.